With the diffusion of information processing devices such as personal computers, printing apparatuses as an image formation terminal also have been widely used. In particular, an ink jet printing apparatus in which ink is ejected from ejection openings to perform a print on a paper or the like has advantages that this apparatus is an non impact type one and uses a printing method causing low noise, this apparatus can perform a printing operation with a high density and a high speed, and this apparatus can easily cope with a color printing, for example. Thus, the ink jet printing apparatus has been increasingly used as a printing apparatus for personal use.
The ink jet printing techniques are widely used as described above and thus have been required to provide a further improved quality of a printed image. In recent years, many print systems for family use that can perform photo printing are available and thus a printed image has been required to have a visual quality equal to that by silver halide photography. In the comparison with silver halide photography, one problem of a granular texture caused in a printed image has been conventionally found. Thus, various structures for reducing this granular texture have been provided.
For example, an ink jet printing apparatus has been known in which, in addition to usual inks of cyan, magenta, yellow, and black, ink of light cyan and light magenta having a lower concentration of color material such as dye is used. This ink jet printing apparatus can use light cyan or light magenta ink in a region having a low printing density to reduce a granular texture. For a region having a high density, cyan or magenta ink having a normal concentration can be used to realize a broader color reproduction range and a smooth tone change.
There is another method for reducing the granular texture by reducing the size of dots formed on a printing medium. In order to realize this method, a technique for reducing an amount of ink droplets ejected from printing heads also has been developed. In this case, in addition to the reduction of an amount of ink droplets, a high-resolution image also can be obtained without lowering the printing speed by arranging more ejection openings with a higher density.
In addition to the technique for reducing the granular texture by focusing on ink to be used, an image processing technique also has been known by focusing on the area coverage modulation method. An ink jet printing apparatus determines whether dots are formed or not with regards to pixels corresponding to a printing resolution to perform a printing based on this determination. For this dot formation, multi-valued image data having predetermined density information is subjected to a quantization processing and is finally converted to binary data. Specifically, a density or tone of an area in a printed image having a size that is macroscopically observed is represented by the number of printed dots and an arrangement thereof. Such a representation of density and tone is generally called as an area coverage modulation. In the area coverage modulation method, a same density can be represented by various dot arrangements. For example, a dot arrangement by the error diffusion method has been known as described in the paper by R. Flold and L. Steinberg titled “Adaptive Algorithm for Spatial Grey Scale” (SDI Int'l Sym. Digest of Tech. papers, 36 to 37 pages (1975)). As a method other than the error diffusion method, a dot arrangement by the ordered dither method also has been known as disclosed in Japanese Patent No. 2,622,429 and Japanese Patent Laid-Open No. 2001-298617. According to these methods, an arrangement of formed dots is visually preferable for which the dispersibility is good and less low frequency components are caused in the space frequency.
Meanwhile, the so-called serial type apparatus in the ink jet printing apparatus widely uses a multi-pass printing method. It is noted that the terms “pass” and “scanning” have the same meaning.
FIG. 1 is a diagram for illustrating the multi-pass printing and schematically showing a printing head and printed dot patterns. In FIG. 1, P0001 denotes a printing head. For simplification of explanation, a printing head is illustrated as having sixteen ejection openings (hereinafter also referred to as nozzles). As shown, a nozzle column is used by being divided to four of the first to fourth nozzle groups each of which includes four nozzles. P0002 denotes a mask pattern in which mask pixels for which a printing is permitted correspondingly to the respective nozzles (print permitting pixels) are shown with black. Mask patterns corresponding to four nozzle groups have complementary relations to each other, and therefore a pattern made by superposing the four mask patterns has 4×4 pixels which are all print permitting pixels. In other words, the four mask patterns are used to complete the printing of a 4×4 area.
P0003 to P0006 denote arrangement patterns of dots to be formed and show how an image is completed by repeating a printing scan. It is noted that the example shown in FIG. 1 shows a case where a so-called solid image is printed in which dots are formed on all of 4×4 pixels. As shown in dot arrangement patterns for the respective scans, a multi-pass printing causes each printing scan to form dots based on binary image data (dot data) generated by using a mask pattern corresponding to each nozzle group. Every time the printing scan is completed, a printing medium is conveyed in a direction shown by the arrow by an amount of a width of one nozzle group. In this manner, an image of each region of the printing medium corresponding to the width of one nozzle group is formed with four times of printing scans.
The multi-pass printing as described above can reduce unevenness of density due to variation in directions and amounts of ink ejection from a plurality of nozzles that may be caused due to manufacture processes and due to errors in a paper conveying operation performed between printing scans to an inconspicuous level.
It is noted that the description with reference to FIG. 1 is related to a four-pass printing as an example in which a single image region is subjected to four printing scans. However, the multi-pass printing itself is not limited to this. Other configurations also may be used such as a two pass printing for completing an image by two times of printing scan, a three pass printing for completing an image by three times of printing scan, or other printings for completing an image by five or more printing scans.
In the multi-pass printing, an arrangement of print permitting pixels of a mask pattern may be changed so that the number of dots printed in the respective printing scan operations can be adjusted and the frequency at which at which a nozzle easily causing problems is used for printing can be reduced. Tat is, in addition to the above elimination of uneven densities or stripe-like density unevenness, other modes can be used depending on various objectives.
As described above, ink jet printing systems in recent years can use various types of inks, execution of multi-pass printing, and a preferred area coverage modulation (binarization method) to output a high-quality and stable image with a high speed.